SALT LAKE CITY (AP) -- Animal-welfare activists are preparing to fight a Utah law that prohibits undercover filming at farm operations.

The Animal Legal Defense Fund is bringing together professors, undercover investigators and a 25-year-old vegetarian who was charged for filming a Utah slaughterhouse. They plan to bring a constitutional challenge to a law that makes it a misdemeanor to trespass on private property to record images or sounds of a livestock operation.

"It just bothered me," Amy Meyer said of a meat-packing plant in the Salt Lake city suburb of Draper along Interstate 15. "I found it shocking what I could see."

She used her digital camera to take a video of a front-end loader dumping a sick cow outside the slaughterhouse. The recording included more graphic scenes and drew a large audience on the Internet. In Utah, Meyer was the first -- and still only -- person to be charged under a law dubbed "ag-gag" that Gov. Gary Herbert signed in March 2012.

Prosecutors eventually dropped the charge of "agricultural operation interference" because Meyer's Feb. 8 video showed that she recorded the operation from the shoulder of a public street. The meat plant has since shielded itself from view, she said.

Slaughterhouses are brutal operations the public should know about and "unfortunately, no laws protect farm animals," said Meyer, an educator at Red Butte Gardens in Salt Lake City.

A sponsor of the Utah law said it protects private property rights.

"It has nothing to do with animals -- it's people trespassing on farms" to make recordings "they go put it on the Internet," said Sen. David Hinkins, R-Orangeville, a cattle operator who also breeds race horses.

"Why pay taxes and let everyone have a free run? I'm sick of it," said Hinkins, who owns several thousand acres of land in central Utah. "This is all about people trespassing on your property. If people can sneak onto anybody's property, then we don't have any rights."

The law undermines free expression, argues Justin Marceau, a law professor at the University of Denver.

"The key is the First Amendment," he said. "Our Founding Fathers didn't like when government tried to control the message."

Utah lawmakers wouldn't think to make it a crime to surreptitiously record abuse at a child-care center, and can't do it just to protect livestock operators, he said.

Iowa has an equally punitive law against filming livestock operations, Marceau said. Other states, including Kansas and Montana, have older laws shielding agricultural operations that have been rarely if ever enforced, he said.

"This will be a really important case," said Marceau, who is helping prepare a lawsuit against the Utah Attorney General's Office. It will list a number of notable animal-rights activists and investigators as plaintiffs, including Daniel Hauff-Lazar.

The Utah law targets people who trespass to record a livestock operation, leave a device behind for recording, or obtain access by false pretense, including by obtaining employment.

The Animal Legal Defense Fund says undercover filming has led to such landmark food safety laws as the Federal Meat Inspection Act and the Pure Food and Drug Act, which help protect the public from "mad cow" disease, E. coli and salmonella.

"These laws have made it virtually impossible to do any whistleblowing or independent investigation," Marceau said.